Conventionally, due to its ease of fluid pouring, paper containers have been widely used for fluid having a spout plug. The spout plug is formed from a spout being positioned on a spout hole part for cutting and opening and incorporating an opening blade and a cap, and is fitted to a top panel having the spout hole part for cutting and opening. A spout hole formed in a paper base material is sealed by a sealing layer, in which the sealing layer sealing the spout hole is cut and opened by the opening blade.
As examples of such a paper container for fluid having a spout plug have been proposed for a number of containers equipped with a cut and opened piece fall prevention mechanism for preventing the cut and opened piece of the sealing layer from falling into the container, since the cut and open piece of the sealing layer is produced when the sealing layer of the spout hole part is cut and opened by the opening blade of the spout plug.
In most cases, the cut and opened piece fall prevention mechanism is provided in the spout plug. In a known example of such a paper container for fluid having a spout plug, the opening blade incorporated in the spout of the spout plug is of a rotary type, and the cut and opened piece fall prevention mechanism is a rotation regulating portion effecting regulation such that the rotation angle of the opening blade in the cutting the sealing layer is less than 360 degrees. Thereby making it possible to effect cutting while leaving a part of the cut and opened piece uncut (see, for example, WO 03/002419).
In another known paper container for fluid having a spout plug, the opening blade incorporated in the spout of the spout plug is of a push-in type, and the cut and opened piece fall prevention mechanism is provided a blade-less portion in a part of the opening blade. Thereby making it possible to effect cutting while leaving a part of the cut and opened piece uncut (see JP 10-194276 A).
Most of the paper containers for fluid having a spout plug formed of a spout incorporating an opening blade and a cap as described above, are gable top type paper containers for fluid, adopted as paper containers for fluid for accommodating contents having a relatively short preservation period. Recently, however, from the viewpoint of ease of pouring, aseptic (germ-free) filling type and brick type paper containers for fluid for accommodating contents having a relatively long preservation period (hereinafter referred to as aseptic/brick type paper containers for fluid) have also come to be required to be equipped with a spout plug formed of a spout containing an opening blade and a cap.
However, in a paper container for fluid for accommodating contents having a long preservation period, the sealing layer is thicker than that of a paper container for fluid for accommodating contents having a short preservation period in order to maintain the long-period preservation property for the contents. As a result, with the opening blade incorporated in the spout of the spout plug, as disclosed in WO 03/002419, which is regulated such that the rotation angle for cutting the sealing layer is less than 360 degrees, the cutting property is rather insufficient, and the cutting is difficult to perform. With the push-in type opening blade contained in the spout of the spout plug as disclosed in JP 10-194276 A, which is regarded as inferior to the rotary type opening blade in terms of cutting property, the cutting is still more difficult to perform.
It has been found through experimentation that this problem can be solved by adopting as the opening blade, a rotary type opening blade, which is superior to the push-in type opening blade in terms of cutting/opening property, and setting the rotation angle of the opening blade in cutting the opened piece to be 360 degrees or more. However, this results in a problem in that the cut and opened piece produced when effecting cutting and opening with the opening blade is cut completely and falls into the container to be mixed into the contents as foreign matter.